Effects of Vapor-Pressure Deficit and Net-Irradiance Calculation Methods on Accuracy of Standardized Penman-Monteith Equation in a Humid Climate
Publication: Journal of Irrigation and Drainage Engineering
Volume 131, Issue 3
Abstract
The effects of some common vapor pressure deficit (VPD) and net irradiance calculation methods on the accuracy of values estimated by using the standardized ASCE Penman-Monteith (ASCE-PM) equation for short grass were examined by comparing the estimated values with measured values in a humid climate. Sensitivity analysis showed 17% and 84% change in the estimated daily values per unit change in the calculated VPD and values, respectively. A total of 12 VPD and 27 calculation methods were examined. Analyses of variance indicated lack of equality in the means of estimated values obtained by different VPD and methods. The percent mean error in the estimated values ranged from for VPD methods and from for methods. On the basis of the coefficient of determination and the standard error of the estimated values, the VPD calculated from saturation vapor pressure , estimated by averaging the at the maximum and minimum daily air temperatures, and actual vapor pressure , estimated by using either the average of minimum and maximum relative humidity or the dew-point temperature, gave more accurate results. Net irradiance estimated by using a regression of relative short-wave solar irradiance, as well as a linear regression on the square root of , resulted in relatively more accurate estimates of than that obtained by methods based on or clear-sky data alone. These results indicate that in a humid climate, some of the VPD and methods have a significant effect on the accuracy of the estimated by using the standardized ASCE-PM equation.
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Acknowledgment
The writers are grateful to the University of Tennessee Agricultural Experiment Station and the Plateau Experiment Station, Crossville, Tennessee, for financial and technical support.
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Information & Authors
Information
Published In
Journal of Irrigation and Drainage Engineering
Volume 131 • Issue 3 • June 2005
Pages: 228 - 237
Copyright
© 2005 ASCE.
History
Received: Dec 5, 2003
Accepted: May 25, 2004
Published online: Jun 1, 2005
Published in print: Jun 2005
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